This invention relates generally to broadband communications, such as cable television systems, and more specifically to optical devices, such as fiber transmitters, within the cable television systems that utilize thermoelectric coolers.
A communication system 100, such as a two-way cable television system, is depicted in FIG. 1. The communication system 100 includes headend equipment 105 for generating forward signals that are transmitted in the forward, or downstream, direction along a communication medium, such as a fiber optic cable 110, to an optical node 115 that converts optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more distribution amplifiers 125 positioned along the communication medium. Taps 130 included in the cable television system split off portions of the forward signals for provision to subscriber equipment 135, such as set top terminals, computers, and televisions. In a two-way system, the subscriber equipment 135 can also generate reverse signals that are transmitted upstream, amplified by any distribution amplifiers 125, converted to optical signals, and provided to the headend equipment 105.
Operators are continuing to revolutionize the conventional network architecture as depicted in FIG. 1, and more recently, have begun to consolidate headends within the network and extend longer fiber optic cable runs as shown in FIG. 2. A network deploying more fiber optic cable than conventional coaxial cable allows a centralized super headend 205 to be shared with sites that may be hundreds of miles away, thereby eliminating several headends 105 throughout the conventional network 100. Emanating from the consolidated headend 205 in all directions are hubs 210 that serve several different sites within the network 200. Fiber equipment enclosed in the hubs 210 distribute the optical signals generated from the headend 205 further through the network 200 until conversion of the optical signals to electrical signals by an optical node 215. The electrical signals are then amplified by an amplifier 220 and continue downstream for the final transmission to the subscriber.
The fiber equipment included in the hubs 210 is generally optical transmitters and receivers, which are contained in racks. Generally, the fiber equipment consumes much of the hub space; therefore, internal airflow is closely monitored to ensure that the fiber equipment does not overheat. In addition to experiencing the heat generated by the fiber equipment, the hubs may become extremely hot in the summer months and extremely cold in the winter months, even though the hubs are traditionally enclosed and somewhat environmentally controlled. As a result, there are external fans for dissipating the heat away from the fiber equipment and cooling devices, such as thermoelectric coolers, designed within the fiber equipment that are used to electrically cool.
By way of example, a thermoelectric cooler (TEC) is used in the optical transmitter to assist in cooling and heating the laser within the transmitter. It is known that controlling the laser temperature to within the standard temperature rating of the laser significantly enhances signal quality of the optical transmission lasers. A laser may be physically attached to the top of the TEC and the whole package may then be hermetically sealed and placed on a heatsink, such as a metal chassis. Functionally, the TEC utilizes current flow to either cool or heat the laser package depending on the environment surrounding the laser. To cool the laser, which is generally the case, current will flow in one direction through the TEC. To heat the laser in the cases of extreme cold, the current will flow in the opposite direction. The amount of cooling or heating is controlled by the magnitude of the current flowing through the TEC.
Conventionally, switching techniques, linear regulation, or a bridge topology can be employed to control the magnitude and direction of current flow through the TEC. Additional details of the conventional regulation of current flow through the TEC are set forth, for example, in a design note by Unitrode DN-76, the teachings of which are incorporated herein by reference.
One conventional example of a device that controls the current through a TEC is depicted in FIG. 3 and further discussed in the Unitrode DN-76 publication, which describes the use of a bridge topology. More specifically, a thermistor 305 is used to detect any temperature fluctuations, and the resulting change in voltage is provided to a pulse width modulated (PWM) controller 310, which is powered by a power supply 315. A bridge topology 320, that includes four field effect transistors (FETs) (not shown), and an LC filter 325 process the signal from the PWM controller 310 to regulate the direction and magnitude of current flow into the TEC 330. Depending upon the magnitude and direction of the current flow, the TEC 330 will either heat or cool the control surface 335. A heatsink 340, such as a metal chassis, absorbs the heat from the surrounding components and the heat generated from the TEC 330 and transfers it away from the control surface 335. An external fan 345 dissipates the transferred heat into the surroundings.
Utilizing the conventional methods, such as a bridge topology 320, of regulating the TEC 330 consumes significant power. More specifically, the device to be cooled consumes power, and this power consumption in turn heats the surrounding area. As a result, additional cooling is often necessary. Thus, what is required is a method of regulating the current flow through a TEC 330 to maximize the power efficiency of the overall device in which it is included, and in addition, to minimize the heat generation that will adversely affect the surrounding components.